Powder actuated tools are well known in the construction field for driving fasteners, such as nails, studs, or anchors, into a relatively hard supporting surface, such as concrete. Such tools utilize a piston for driving the fastener. The piston is typically driven by an explosive blank cartridge. Combustion gases generated from the cartridge drive the piston from a breechward position to a muzzleward position to drive the fastener into the supporting surface. Typically, tools of this type cannot be fired unless the muzzle of the tool is pushed against the surface into which the fastener is to be driven.
Due to the environment in which the tools are used, and their mode of operation, even the best made tools are subject to disabling wear and breakage. Such wear and breakage can be expensive since the entire tool must be replaced. At the very least, the user is without the use of the tool while it is being repaired.
The powder actuated tool shown in U.S. Pat. No. 4,493,376, granted Jan. 15, 1985 has a searing mechanism which uses a sear pawl which is spring-biased in a bore in the firing pin. The sear pawl is biased toward a searing position wherein the pawl engages a blocking surface on a slidable breech block in which the firing pin is slidably mounted. Both the breech block and firing pin are separately biased muzzlewardly in the tool. The sear pawl is movable in its bore along a path of travel which is disposed at a 90 degree angle relative to the direction of movement of the firing pin and breech block. When the tool is cocked, the breech block blocking surface will be urging the lower end of the sear pawl in a breechward dirction. At the same time, the firing pin spring will be urging the upper end of the sear pawl (which is disposed in the firing pin bore) in the muzzleward direction. Since the sear-releasing movement occurs in a direction perpendicular to the direction of movement of the firing pin, the aforesaid counterdirectional urging of the sear pawl actually weakens the searing ability of the sear pawl. In order to ensure that the sear will not accidentally release, the diameters of the sear pawl and the firing pin sear bore must be very closely held so that the sear pawl cannot pivot in the bore sufficiently to release the firing pin. In automatic tools which utilize a strip or disk charge magazine, the bias on the firing pin is constant, even when the tool is not cocked, thus the tendancy to pivot the sear pawl is constant. It will be appreciated that extensive use of the tools will exacerbate the aforesaid tendency to prematurely release the sear pawl. In fact, tools have accidently fired when not even cocked because of premature release of the sear pawl. Such a problem requires a solution.